add index for sis_batch_errors

fixes CORE-2234

test plan
 - db:migrate should work

Change-Id: If5ef6beadcb9f5fb32cf47cf0a8a35b542b82480
Reviewed-on: https://gerrit.instructure.com/174847
Reviewed-by: James Williams <jamesw@instructure.com>
Tested-by: Jenkins
QA-Review: Rob Orton <rob@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>

db/migrate/20181210175230_add_created_at_index_to_sis_batch_errors.rb

@@ -0,0 +1,25 @@
+#
+# Copyright (C) 2018 - present Instructure, Inc.
+#
+# This file is part of Canvas.
+#
+# Canvas is free software: you can redistribute it and/or modify it under
+# the terms of the GNU Affero General Public License as published by the Free
+# Software Foundation, version 3 of the License.
+#
+# Canvas is distributed in the hope that it will be useful, but WITHOUT ANY
+# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+# A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along
+# with this program. If not, see <http://www.gnu.org/licenses/>.
+
+class AddCreatedAtIndexToSisBatchErrors < ActiveRecord::Migration[5.1]
+  tag :postdeploy
+  disable_ddl_transaction!
+
+  def change
+    add_index :sis_batch_errors, :created_at, algorithm: :concurrently
+  end
+end